While developing a standardized fluids assessment covering buoyancy and pressure, we discovered deficiencies in student understanding of density. In particular, many college students do not recognize that density is a fixed property of a solid substance, such as aluminum or gold. We added questions to our diagnostic exam to probe the extent of student difficulties. In one of our questions, only 50-60% of students recognize that the density of gold is a fixed value. When similar questions from an existing diagnostic [1] are used, however, 85-90% of students correctly identify the density of a piece of wood and of a diamond as fixed values. In this paper we discuss the differences between these questions and how those differences affect student responses.